Accumulating evidence from our group and others suggest that bone marrow (BM) sinusoidal endothelial cells (SECs) represent a dynamic "vascular niche", which may provide the cellular platform for the reconstitution of hematopoiesis after myelosuppression. Using technical advances in bone marrow (BM) preparation, we have recently established a comprehensive phenotypic and functional signature of BM SECs at steady state and during hemangiogenic regeneration. We have recently shown that, after moderate to severe myelosuppression rapid regeneration of the regressed SECs is essential for engraftment and replenishment of the transplanted long-term hematopoietic stem cells (LT-HSCs) and reconstitution of hematopoiesis. Most likely, transplanted HSC and their lineage committed hematopoietic progenitor cells (HPCs) by releasing of neo- angiogenic factors contribute to the regeneration of SEC. However, the precise mechanism by which angiogenic factors released by the pro-angiogenic hematopoietic cells, such as CXCR4+VEGFR1+ myeloid and megakaryocytic progenitors cells support reconstruction of the SECs is not known. The broad long-term objective of this proposal is to identify the molecular pathways and to define the mechanism whereby angiogenic factors, specifically the VEGF-A isoforms, PlGF and SDF1 elaborated by specific subsets of the hematopoietic cells support assembly and remodeling of BM's "Vascular Niche", thereby supporting reconstitution of HSCs and hematopoiesis after myelosuppression. Therefore, we hypothesize that within BM, the VE- cadherin+VEGFR2+VEGFR3+Sca1- SECs establish a vascular niche, which is a dynamic cellular microenvironment essential for the reconstitution of HSC, and hematopoiesis after myelosuppression. Regenerating pro-angiogenic CXCR4+VEGFR1+ hematopoietic cells through release of VEGF-A SDF-1, and as yet unrecognized angiogenic factors accelerate regeneration of the SECs thereby accelerating the reconstitution of the LT- HSCs and hematopoiesis. This hypothesis will be tested through: 1) determining the mechanism by which pro-angiogenic hematopoietic cells by elaborating VEGF-A and SDF-1 support the regeneration of regressed SECs thereby reconstituting LT-HSCs and hematopoiesis: 2) assessing the relative contribution of preexisting CXCR4+ endothelial cells versus transplanted CXCR4+VEGFR1+ hematopoietic cells to the revascularization of the ischemic limbs and 3) evaluating the physiological significance of enforced expression of angiogenic factors in accelerating the regeneration of BM SECs and reconstitution of hematopoiesis. We anticipate that understanding the mechanism by which angiogenic factors regulate hematopoiesis and HSC self-renewal will offer new strategies to treat BM failure states, including aplastic anemia, myelodysplastic syndromes and accelerate BM reconstitution after chemotherapy, irradiation and transplantation. PUBLIC HEALTH RELEVANCE: We hypothesize that bone marrow's vascular niche is a dynamic cellular microenvironment that is essential for the maintenance and reconstitution of hematopoiesis after myelosuppression. Regenerating CXCR4+VEGFR1+ hematopoietic cells through release of angiogenic factors, including VEGF-A, PlGF, SDF-1 and FGF-2 support regeneration of the sinusoidal endothelial cells into functional vascular niche thereby accelerating the restoration of hematopoietic stem cells and reconstitution of hematopoiesis. We anticipate that understanding the mechanism by which angiogenic factors regulate the reconstruction of the vascular niche in the bone marrow will offer new strategies to treat hematopoietic failure states, including aplastic anemia, myelodysplastic syndromes and accelerate BM reconstitution after chemotherapy, irradiation and bone marrow transplantation.